The field of this invention relates to plugs inserted from the surface into a wellbore, generally used for fluid or cement displacement, wherein the plug comprises a size variation capability to sealingly conform to tubular size changes as it is propelled downhole.
Wiper plugs are frequently used in completions such as when a liner is hung in casing and needs to be cemented. The cement is generally pumped downhole with the wiper plug in front. The wiper plug is launched from a holder at the surface and may need to travel through a variety of diameters before it comes to the receptacle where it xe2x80x9cbumpsxe2x80x9d to give the surface personnel an indication of its arrival. In some applications, a wiper plug is used to separate well fluids pumped behind the cement to further displace the cement. In this application references to plug or wiper plug is intended to encompass drill pipe darts or plugs
To avoid having to inventory a large variety of sizes for different applications the wiper plugs of the prior art had multiple fins so that at any given time one of the fins would sealingly engage the wall so the plug would be pumped further downhole. FIGS. 1 and 2 are illustrative of a prior art wiper plug. The wiper plug 10 is shown schematically just as it is about to be inserted into a drill pipe 12. There are three rows of fins 14, 16, and 18 of differing diameters. Again, this is done so one size wiper plug 10 fits many different applications. Depending on the application one or more of the fins need to be folded over themselves to such a degree that a xe2x80x9cfloweringxe2x80x9d or xe2x80x9cpetalingxe2x80x9d effect shown in FIG. 2 can occur. This effect creates a plurality of longitudinal troughs 20 when a fin is compressed. In a typical application the elastomer material used to make the fin has too little memory and fails to completely reassume its original shape when allowed to expand as the wiper plug 10 reaches a larger tubular, after it is launched. The problem this brings on is that cement or other fluids can pass around wiper plug 10 in the troughs that remain after reaching the bigger tubular. The retention of such troughs 20 also prevents a good circumferential seal from occurring at the interface of the fin extremity and the inner tubular wall.
It is an objective of the present invention to solve this problem so as to improve the performance of wiper plugs downhole. It is another objective to make the fin portion of a wiper plug flexible, to accommodate a variety of sized openings, even in a single run. Another object is to be able to control the amount of contact force against varying tubular inside diameters on a real time basis as the wiper plug progresses downhole. These and other objectives will become more clear to those skilled in the art from a review of the preferred embodiment, described below.
The following patents represent plugs, packers and other downhole devices that have been used downhole: U.S. Pat. Nos. 3,100,534; 4,676,310; 4,729,429; 4,341,272; 3,690,375; 3,575,238; 2,294,521; and 1,639,079.
A wiper plug for downhole use is disclosed. It features an inflatable structure that allows it to ride inside tubulars that change or gradually vary in inside diameter. In a preferred embodiment the bladder is actuated by fluid displaced by a biased piston. The piston is capable of moving in opposite directions to allow original insertion into a launcher and subsequent bladder expansion. In another embodiment, the piston can be fluid driven in opposed directions by a pump and an on board control system which can regulate, on a real time basis, the contact pressure of the bladder to a predetermined level or range, as the bladder encounters varying interior wall diameters of the tubular string or associated equipment.